Method 1: Knock on different pieces of the car. A painted piece will have base(putene) and would sound different
Method 2: Sit beside the car and painted piece will have bubbles or a different texture.
Method 3: At night repainted piece will have different shade than the original one. This method is the simplest one(especially incase of silver cars) and even a kid can tell whether a car is painted from anywhere or not using this method